Lithofacies and petrochemical characterization of volcano-sedimentary sequence of Chandil Formation around Kharidih-Bareda area, Seraikela-Kharsawan District, Jharkhand: implications for uranium mineralization

Mesoproterozoic Chandil Formation (ca. 1600 Ma) of North Singhbhum Mobile Belt record numerous features of felsic volcaniclastics and felsic to intermediate volcanics preserved in the central sector of the fold belt around Kharidih-Bareda area, Seraikela-Kharsawan district, Jharkhand. The felsic volcanic rocks exhibit flow bands, autoclasts and layering of crystal mushes revealing viscous nature of eruptives. The volcaniclastic sediments comprise of significant proportion of volcanic epiclasts and accidental lithic fragments. These volcaniclastics have been categorized into five prominent lithofacies viz, stratified lapilli tuff, banded tuff, tuff with penecontemporaneous deformation, welded lapilli stones, vitric tuff and volcanic bombs by field and petrographic studies of outcrops and subsurface borehole cores. The welded lapilli tuffs display fiamme and eutaxitic texture. Interlayering of the volcaniclastics, which are most often pyrite-rich, with psamo-pelitic lithology like carbonaceous phyllite, variegated phyllite, quartzite and minor limestone is suggestive of marine euxenic depositional environment. Petrographic study of the volcaniclastics indicated presence of glass shards, garnet phenocrysts, spherules of tremolite, ovoid to lenticular accretionary lapilli along with devitrified glassy material. Compositionally these felsic volcanics and volcaniclastics are rhyodacitic to andesitic in nature with peraluminous to meta aluminous in character. A/CNK values vary from 0.52 to 2.42 in felsic volcanics and from 0.12 to 1.63 in volcaniclastics. Signatures of arc magmatism is indicated by low concentration of HFS elements such as Nb (5-17 ppm), Ga (11-17 ppm) and Y (5-28 ppm). Elevated intrinsic content of uranium (3-8 ppm), Th/U ratio ranging from 1.2 to 13.2, presence of metamict allanite and zircon in volcanics and volcaniclastics reveal their suitability as a prospective source for search of uranium mineralization. The volcanic-volcaniclastic-clastic association of the Chandil Formation provides an ideal situation where provenance and province both are available. Thus, suitable litho-structural locales such as the concealed shear zones sympathetic to the Dalma thrust and South Purulia Shear Zone within the volcano-sedimentary package of Chandil Formation may be targeted as preferable sites for locating concealed uranium mineralization.

Chapter 5: The Low-Grade, Neoproterozoic, Vein-Style, Carbonaceous Phyllite-Hosted Paracatu Gold Deposit, Minas Gerais, Brazil

The Paracatu deposit in Brazil is a shallowly dipping, bulk-tonnage, low-grade, vein-style orogenic Au orebody hosted in very strongly deformed Neoproterozoic carbonaceous phyllite of the southern Brasília fold belt. At regional to district scales, the gold orebody lies along the eastern, hanging-wall edge of a major thrust of the ~630 Ma Brasiliano orogeny. This thrust cuts through a facies transition between clastic-dominated rocks of the Canastra Group and carbonate-dominant rocks of the Vazante Group, deposited at ~1000 Ma in a rift to passive-margin environment on the flank of the São Francisco craton. At the same scales, the footwall of this major thrust system hosts numerous structurally controlled zinc deposits including Vazante and Morro Agudo. At Paracatu, ore genesis occurred primarily by the formation of early tectonic quartz sulfide-carbonate veins, prior to substantial ductile deformation (boudinage), local physico-chemical reworking of these veins, and redistribution of some gold. Structural, geochemical, and isotopic data indicate a strong influence of the local rocks (cm to 100-m scales) on many ore ingredients, and the quartz and carbonate in ore veins were most likely derived locally (cm to m scales). However, the coassociation of gold and arsenic with the boudinaged veins and a major thrust, and the absence of metal enrichments normally associated with syngenetic metalliferous black shales, supports a model of far-field derivation of gold within this metasedimentary package (km to 10-km scales). Transport of metal-bearing fluids toward a favorable structural and chemical site during thrusting and orogenesis was possibly focused, during precipitation to ore grades, by the position of transverse structures in the basement, which also influenced deposition of the adjacent zinc deposits. Successful mining of the low-grade resource was initially favored by the subhorizontal orebody geometry and weathering characteristics, and subsequently by high production rates from the 100-m-thick mineralized zone.

The thrust contact between the Canastra and Vazante groups in the Southern Brasília Belt: structural evolution, white mica crystallinity and implications for the Brasiliano orogeny

ABSTRACT: Two regional thrust-sheets of Neoproterozoic metasedimentary rocks occur in the Southern Brasília Belt, northwest Minas Gerais. The lower one comprises the Vazante Group, that is formed in the studied area, from base to top, by the Serra do Garrote (metapelites interlayered with carbonaceous phyllite), Serra do Poço Verde (beige to pink stromatolitic metadolomite with interlayered greenish slates), Morro do Calcário (gray stromatolitic metadolomite interlayered with gray slates) and Serra da Lapa (phyllite with dolarenitic lenses interlayered with slates) formations. The upper thrust sheet consists of the Canastra Group (Paracatu formation): laminated sericite phyllites and carbonaceous phyllites interlayered with quartzite. The Braziliano orogeny resulted in four phases of contractional deformation, associated with low-grade metamorphism. The first two (D1 and D2) are ductile, while the third and fourth ones (D3 and D4) are brittle-ductile. D1 developed a slaty S1 cleavage subparallel to the primary layering, with shallow to steep dips to NW. D2 developed a crenulation cleavage (S2) that dips moderately to NW and is associated with tight to isoclinal folds. D3 and D4 phases developed crenulations and open folds and kink bands. S3 dips steeply to NW, while S4 has moderate to steep dips to NE and SW. White mica crystallinity (Kübler index) measurements in metapelites indicate that both the Canastra and Vazante groups reached anchizone/epizone conditions, and metamorphic discontinuities along thrusts indicate that the peak of metamorphism is pre or syn-thrusting.

ESTUDO DE INCLUSÕES FLUIDAS EM VEIOS DE QUARTZO E CARBONATO (± SULFETOS-SULFOSSAIS) DO DEPÓSITO CARVOARIA VELHA, LINEAMENTO AURÍFERO CÓRREGO DO SÍTIO, SANTA BÁRBARA, QUADRILÁTERO FERRÍFERO, MG

O Lineamento Aurífero Córrego do Sítio corresponde a uma zona de cisalhamento dúctil com um trend NE-SW controlandoquatorze depósitos de ouro, no qual se inclui o depósito Carvoaria Velha. A mineralização de ouro se hospeda em rochas arqueanas dogreenstone belt Rio das Velhas, estas correspondem a filitos e xistos finos alterados a quartzo, carbonato e sericita. O estilo demineralização principal ocorre em veios de quartzo-carbonato-sulfetos (±sulfossais). Os tipos de veios são classificados como: Veios V1 –quartzo-ankerita-sulfetos-berthierita-ouro – são paralelos à principal foliação Sn e estão localmente mineralizados em ouro. Veios V2 –quartzo-ankerita-pirita – são veios desenvolvidos no plano de clivagem de crenulação Sn+1, extensionais e raramente mineralizados. VeiosV3 – quartzo-ankerita – são veios de preenchimento de fraturas Sn+3, geralmente não contêm sulfetos e sulfossais. Veios V4 – quartzocalcita– são veios restritos aos diques e sills metamáficos, são extensionais e sem orientação preferencial. Os minerais-minério sãoarsenopirita, pirita, berthierita e pirrotita. Microssonda eletrônica revela a presença de fases metálicas ricas em As, Sb e Co (boulangerita,tetraedrita, cobalita e ullmanita), que sobrecrescem aos minerais-minério. Dados de inclusões fluidas caracterizam fluido mineralizadorcom composição H2O–CO2±CH4–NaCl, com traços de N2, podendo conter outros cátions além de Na+, como Mg2+, Ca2+ e Fe2+. Estes fluidosteriam pelo menos dois estágio de evolução: i) fluido inicial aprisionado em quarto fumê, com salinidade moderada (~8.5% em peso deNaCl eq.) e temperatura mínima de 280±17oC; e ii) fluido tardio, representado por fluido aprisionado em quartzo recristalizado, menossalino (~4.6% em peso de NaCl eq.) e temperatura mínima de 315±33oC. Os dados obtidos correspondem a valores típicos de fluidometamórfico característico de depósitos tipo orogenic gold.Palavras-chave: mineralização aurifera, inclusões fluidas, Quadrilátero Ferrífero, alteração hidrotermal ABSTRACTFLUID INCLUSION STUDIES IN QUARTZ-CARBONATE(± SULFIDES-SULFOSALTS) VEINS FROM CARVOARIA VELHA DEPOSIT,CÓRREGO DO SÍTIO GOLD LINEAMENT, SANTA BÁRBARA, QUADRILÁTERO FERRÍFERO, MG. The Córrego do Sítio Lineament is defined as a16 km long, NE-SW trending ductile shear zone which controls fourteen deposits, one of them is Carvoaria Velha deposit. Themineralization is hosted in Archean rocks of Rio das Velhas greenstone belt, represented for carbonaceous phyllite and schists withcarbonate, sericite, sulphide-sulphosalt and silica alteration. The main gold mineralization styles at the Córrego do Sítio lineament arequartz-carbonate-sulphide±sulphosalts veins of varied distribution. They are classified as: V1 veins – quartz-ankerite-pyrite-berthierite-gold– parallel to the main regional foliation Sn, and usually mineralized in gold; V2 veins – quartz-ankerite-pyrite – developed at the crenulationcleavage Sn+1, extensional and rarely gold mineralized; V3 veins – quartz-ankerite – filling Sn+3 fractures, usually free of sulphides andsulphosalts; V4 veins – quartz-calcite – of restricted occurrence in metamafic dykes and sills; they crosscut the metasedimentary sequence,are extensional and display no preferential orientation. The most common sulphides in all vein types are arsenopyrite, pyrite andpyrrhotite. Microprobe studies show the presence of metallic phases rich in Sb, Pb and Co, such as stibnite, ullmanite, tetrahedrite, galena,cobaltite and, commonly overgrowing. Based on the microthermometric data, the hydrothermal fluid is interpreted to reflect at least twoevolved stages: i) an early fluid stage, trapped in smoky quartz, of moderate ~8.5 eq. wt% NaCl salinity, and minimal trapping temperatureof 280±17oC; and ii) a late-stage fluid trapped in recrystallized quartz with lower ~4.6 eq. wt% NaCl salinity, and a minimal trappingtemperature of 315±33oC. These values are typical of metamorphic fluid characteristic of orogenic gold deposits.Keywords: gold mineralization, fluid inclusions, Quadrilátero Ferrífero, hydrothermal alteration

Tectonic framework of the upper Paleozoic and lower Mesozoic Alava sequence: a revised view of the polygenetic Taku terrane in southern southeast Alaska

Fragments of upper Paleozoic and lower Mesozoic metavolcanic and metasedimentary sequences of the Taku terrane are exposed discontinuously along a narrow belt in southeast Alaska and form a distinct lithostratigraphic package in the Ketchikan area called the Alava sequence. Crinoidal and argillaceous marble, carbonaceous phyllite, argillite, mafic flows, pillow breccia, pyroclastic tuff, and quartzite characterize the sequence. These strata are unconformably overlain by Upper Jurassic to Lower Cretaceous fine- to coarse-grained epiclastic rocks of the Gravina sequence. The upper Paleozoic part of the Alava sequence may be correlative with the Yukon–Tanana terrane, whereas the Middle and Upper Triassic portion of the Alava sequence may represent a metamorphic vestige of the Stikine terrane. Both parts are now exposed on the western flank of the Coast Plutonic Complex, in contrast with their correlatives to the east. These relations suggest that the Stikine and Alexander terranes were juxtaposed prior to deposition of the Gravina sequence. The western boundary between rocks of North American affinity and allochthonous ensimatic crustal fragments of the Alexander and Wrangellian terranes lies west of the Coast Plutonic Complex.